Semiconductor memory system

ABSTRACT

According to one embodiment, a semiconductor memory system includes a substrate, a plurality of elements and an adhesive portion. The substrate has a multilayer structure in which wiring patterns are formed, and has a substantially rectangle shape in a planar view. The elements are provided and arranged along the long-side direction of a surface layer side of the substrate. The adhesive portion is filled in a gap between the elements and in a gap between the elements and the substrate, where surfaces of the elements are exposed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of priorityunder 35 U.S.C. §120 from U.S. application Ser. No. 14/324,683, filedJul. 7, 2014, which is a continuation of U.S. application Ser. No.13/418,619, filed Mar. 13, 2012, which is based upon and claims thebenefit of priority under 35 U.S.C. §119 from Japanese PatentApplication No. 2011-058140, filed on Mar. 16, 2011; the entire contentsof each of which is incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a semiconductor memorysystem.

BACKGROUND

In a process for manufacturing a semiconductor device, a method has beenemployed, using a semiconductor memory system mounting nonvolatilesemiconductor storage elements such as an NAND flash memory on asubstrate in which a connector is formed. Also, in the semiconductormemory system, in addition to the nonvolatile semiconductor storageelements, high-speed semiconductor storage elements and a controllerthat controls the high-speed semiconductor storage elements and thenon-volatile semiconductor storage elements are mounted.

In such a semiconductor memory system, there is a case where thesubstrate shape and size are restricted according to the usageenvironment and the standard, for example, there is a case where asubstrate having a rectangle shape in a planar view is used. Also,according to a miniaturization request to a recent semiconductor memorysystem, a substrate tends to become thinner. In the case of using such athinned rectangle-shaped substrate, the warpage of substrate isrequested to be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a configuration example of a semiconductormemory system according to a first embodiment;

FIG. 2A is a plan view of a schematic configuration of a semiconductormemory system;

FIG. 2B is a plan view of a schematic configuration of a semiconductormemory system as other examples;

FIG. 3A is a side view of the semiconductor memory system shown in FIG.2A;

FIG. 3B is a side view of the semiconductor memory system shown in FIG.2B;

FIG. 4 is a view of a substrate layer configuration;

FIG. 5 is a view of the wiring density of each substrate layer;

FIG. 6 is a view of a wiring pattern formed on a rear surface layer (orthe eighth layer) of the substrate;

FIG. 7 is a view of the wiring density of each substrate layer as acomparison example;

FIG. 8 is a view for explaining the line width and interval of thewiring pattern formed on the rear surface layer (or the eighth layer) ofthe substrate;

FIG. 9 is a view of an adhesive portion in which the gap between NANDmemories is filled;

FIG. 10 is a view of slits formed in a seventh layer of the substrate;

FIG. 11 is a view of a substrate layer configuration provided in asemiconductor memory system according to a second embodiment;

FIG. 12 is an external perspective view of a holding member used in acarrying method of a semiconductor memory system according to a thirdembodiment;

FIG. 13 is a cross-sectional view of a state where the holding membershown in FIG. 12 is put in a box;

FIG. 14 is a front view of a holding member according to a variationexample of the third embodiment;

FIG. 15 is a view of a state where a moving portion of the holdingmember shown in FIG. 14 is opened; and

FIG. 16 is a view of a configuration example of a SATA interface.

DETAILED DESCRIPTION

In general, according to one embodiment, the semiconductor memory systemincludes a substrate, nonvolatile semiconductor storage elements and anadhesive portion. The substrate has a multilayer structure in whichwiring patterns are formed, and has a substantially rectangle shape in aplanar view. The nonvolatile semiconductor storage elements are providedand arranged along the longitudinal direction on a surface layer side ofthe substrate. The adhesive portion is filled in a gap between thenonvolatile semiconductor storage elements and in a gap between thenonvolatile semiconductor storage elements and the substrate, where thesurfaces of the nonvolatile semiconductor storage elements are exposed.

Exemplary embodiments of the semiconductor memory system will beexplained below in detail with reference to the accompanying drawings.The present invention is not limited to the following embodiments.

FIG. 1 is a block diagram of a configuration example of a semiconductormemory system according to a first embodiment. A semiconductor memorysystem 100 is connected to a host apparatus (hereinafter abbreviated to“host”) 1 such as a personal computer and a CPU core via a memoryconnection interface such as a SATA interface (ATA I/F) 2, and functionsas an external memory of the host 1. Examples of the host 1 include apersonal computer CPU and a CPU of an imaging apparatus such as a stillcamera and a video camera. Also, the semiconductor memory system 100 cantransmit and receive data with a debug device 200 via a communicationinterface 3 such as an RS232C interface (“RS232C I/F”).

The semiconductor memory system 100 is provided with NAND-type flashmemories (hereinafter abbreviated to “NAND memories”) 10 as nonvolatilesemiconductor storage elements (elements), a drive control circuit(element) 4 as a controller, a volatile DRAM 20 as a high-speedsemiconductor storage element (element) that can perform faster storageoperations than the NAND memories 10, a power supply circuit 5, a statedisplay LED 6 and a temperature sensor 7 that detects the temperatureinside the drive. The temperature sensor 7 directly or indirectlymeasures the temperature of the NAND memories 10, for example. If ameasurement result by the temperature sensor 7 is equal to or higherthan a predetermined temperature, the drive control circuit 4 restrictsthe writing of information into the NAND memories 10 and suppresses anincrease in temperature.

Here, as a nonvolatile semiconductor storage element, a lamination-typeNAND flash memory or an ReRAM (Resistive Random Access Memory) may beused. Also, as a high-speed semiconductor storage element, a nonvolatileMRAM (Magnetoresistive Random Access Memory) may be used. The MRAM mayhave a magnetic screening portion to suppress intrusion of magnetismtherein. Also, in a case where the MRAM itself does not have themagnetic screening portion, a periphery coverer (not shown) to suppressa magnetic entry by covering the periphery of the MRAM or the NANDmemories 10 may be provided.

The power supply circuit 5 generates a plurality of different internaldirect-current power supply voltages from an external direct-currentpower supply voltage supplied from a power supply circuit on the side ofthe host 1, and supplies these internal direct-current power supplyvoltages to each circuit inside the semiconductor memory system 100.Further, the power supply circuit 5 detects the rise of the externalpower supply, generates a power-on reset signal and supplies it to thedrive control circuit 4.

FIG. 2A is a plan view of a schematic configuration of the semiconductormemory system 100. FIG. 3A is a side view of the semiconductor memorysystem 100 shown in FIG. 2A. The power supply circuit 5, the DRAM 20,the drive control circuit 4 and the NAND memories 10 are mounted on asubstrate 8 in which wiring patterns are formed. The substrate 8 has asubstantially rectangle shape in a planar view. On the side of one shortside of the substrate 8 having a substantially rectangle shape, aconnector 9 connectable to the host 1 is provided. The connector 9functions as the SATA interface 2 and the communication interface 3described above. The connector 9 functions as a power supply inputtingportion that supplies the power supply input from the host 1 to thepower supply circuit 5. The connector 9 may be an LIF connector, forexample. Also, in the connector 9, a slit 9 a is formed in a positionshifted from the center position along the short-side direction of thesubstrate 8, and fits with, for example, a projection (not shown)provided on the side of the host 1. By this means, it is possible toprevent the semiconductor memory system 100 from being attachedreversely.

The substrate 8 has a multilayer structure formed by layering syntheticresins and has eight-layer structure, for example. Here, the number oflayers of the substrate 8 is not limited to eight. FIG. 4 is a view of alayer configuration of the substrate 8. In the substrate 8, a wiringpattern is formed in various forms as a wiring layer 8 b in the surfaceor inner layer of each layer (insulating film 8 a) formed with syntheticresins. The wiring pattern is formed with a copper, for example. Via thewiring pattern formed on the substrate 8, the power supply circuit 5,the DRAM 20, the drive control circuit 4 and the NAND memories 10mounted on the substrate 8 are electrically connected to each other.Also, the surface (on the first layer side) and rear surface (on theeighth layer side) of the substrate are covered by a solder resist 8 cas a protection film.

FIG. 5 is a view of the wiring density of each layer of the substrate 8.Here, the first to fourth layers formed on the surface layer sidecompared to a center line 30 (see FIG. 4) of the layer configuration ofthe substrate 8 are collectively referred to as “upper layer” and thefifth to eighth layers formed on the rear surface layer side compared tothe center line 30 are collectively referred to as “lower layer.”

As shown in FIG. 5, the wiring layer 8 b formed in each layer of thesubstrate 8 functions as a signal layer for signal transmission andreception, a ground and a plane layer which is a power line. Then, thewiring density of the wiring pattern formed in each layer, that is, theratio of a wiring layer to the surface area of the substrate 8 is asshown in FIG. 5.

In the present embodiment, the eighth layer functioning as a ground isformed as a meshed pattern wiring layer instead of a plane layer, sothat the wiring density is suppressed to 30% to 60%. Here, in the entireupper layer of the substrate 8, the wiring density is 60%. Therefore, byforming a wiring pattern with the wiring density of approximately 30% ofthe eighth layer, it is possible to set the wiring density of the entirelower layer to 60% and make the wiring density of the entire upper layerand the wiring density of the entire lower layer substantially equal.Also, by adjusting the wiring density of the eighth layer within a rangeof approximately 30% to 60%, the wiring density is made substantiallyequal to the wiring density of the entire upper layer.

FIG. 6 is a view of a wiring pattern formed on a rear surface layer (orthe eighth layer) of the substrate 8. As shown in FIG. 6, the wiringpattern is formed in a meshed pattern in the rear surface layer (or theeighth layer) of the substrate 8. Thus, by forming the eighth layer ofthe substrate 8 with a net wiring layer, the wiring density is keptlower, compared to a case where a plane layer is formed.

A wiring layer formed in the rear surface layer is also requested tofunction as a shield layer to alleviate the influence of noise which isleaked from the semiconductor memory system 100 and given to otherapparatuses. FIG. 8 is a view for explaining the line width and intervalof the wiring pattern formed on the rear surface layer (or the eighthlayer) of the substrate. As shown in FIG. 8, a net wiring is formed onthe eighth layer of the substrate 8, where line width L is 0.3 mm andline interval S is 0.9 mm. In the meshed pattern wiring formed in thisway, open width W is 0.9×√2=1.27 mm.

For example, a shield effect to noise of radio frequency such as a SATAfundamental wave of 3 GHz is as follows. First, from C=f×λ×√ε, the ½wavelength (λ/2) of second harmonic wave of the SATA fundamental wave iscalculated. Here, “C” represents the light speed of 3.0×10⁸ m/s.Further, “f” represents frequency of the second harmonic wave and has avalue of 6.0×10⁹ Hz. Further, “ε” represents a relative permittivity of4.6.

According to the above conditions, λ is 23.3 mm and the ½ wavelength(λ/2) is 11.7 mm. That is, the ½ wavelength (λ/2) is about ten times ofopen width W (1.27 mm). Also, λ/20 is 1.2 mm, which is substantiallyequal to open width W, and therefore the shield effect is about −20 dB.

FIG. 9 is a view of an adhesive portion in which the gap between NANDmemories is filled. As shown in FIG. 9, an adhesive portion 31configured with synthetic resin materials is filled in a gap between theNAND memories 10 and the substrate 8, which bonds the NAND memories 10and the substrate 8. Further, the adhesive portion 31 is partiallyprotruded from the gap between the NAND memories 10 and the substrate 8.The protruded part is filled in the gap between the NAND memories 10arranged along the long-side direction of the substrate 8. Therefore,the adhesive portion 31 bonds the NAND memories 10 on their sidesurfaces. Here, the adhesive portion 31 is protruded to the extent thatit does not exceed the height of the NAND memories 10, and therefore thesurfaces of the NAND memories 10 are exposed. Also, in FIG. 9, althoughthe adhesive portion 31 is filled to around the intermediate part of theheight of the NAND memories 10, it may be filled to a lower part, and anessential requirement is that the adhesive portion 31 touches adjacentNAND memories 10. It is natural that the adhesive portion 31 may befilled between the NAND memories 10 to be higher than the height shownin FIG. 9. Also, the adhesive portion 31 is filled between thecontroller 4 and the NAND memories and between the controller 4 and theDRAM 20.

FIG. 2B is a plan view of a schematic configuration of the semiconductormemory system 100 as other examples. FIG. 3B is a side view of thesemiconductor memory system 100 shown in FIG. 2B. Thus, the adhesiveportion 31 may be filled between the NAND memory 10 and the RAM 20.

FIG. 10 is a view of slits formed in the seventh layer of the substrate8. FIG. 10 shows a state seen from the rear surface layer side of thesubstrate 8 and shows the eighth layer in an abbreviated manner.Further, the NAND memory 10 mounted on the surface layer side isrepresented by dotted line. In the seventh layer of the substrate 8, aplane layer is formed as a wiring layer. As shown in FIG. 10, in theseventh layer of the substrate 8, the wiring pattern as a plane layer isformed over the substantially entire seventh layer, in part of whichslits 32 (i.e., parts in which the wiring layer is not formed) areprovided. The slits 32 are provided in parts facing the gaps between theNAND memories 10, in the wiring pattern formed over the substantiallyentire seventh layer.

FIG. 7 is a view of the wiring density of each substrate layer as acomparison example. As shown in the comparison example of FIG. 7, in aconventional substrate, the eighth layer is formed with a plane layerand therefore the wiring density is approximately 90%. Accordingly, thewiring density of the lower layer is 75%, which increases the differencefrom the wiring density of the upper layer (approximately 60%) Byvarying the wiring density, the ratio between the insulating film 8 a(i.e., synthetic resin) and the wiring part (i.e. copper) in the entireupper layer of the substrate 8 differs from the ratio between thesynthetic resin and the copper in the entire lower layer of thesubstrate 8. By this means, a thermal expansion coefficient variesbetween the upper layer and the lower layer of the substrate 8. By thisthermal expansion coefficient difference, depending on a temperaturechange in the substrate 8, the warpage having a convex shape (i.e.,upper convex shape in FIG. 3) is likely to occur on the surface layerside along the long-side direction of the substrate 8. Such atemperature change is likely to occur in the production process of thesemiconductor memory system 100. Also, since a recent semiconductormemory system is requested to be miniaturized, the substrate 8 tends tobecome thinner, and therefore such a warpage is likely to occur.

On the other hand, in the present embodiment, the wiring density of theeighth layer is adjusted to a range of approximately 30% to 60% to makethe wiring density of the entire upper layer and the wiring density ofthe entire lower layer substantially equal, so that the thermalexpansion coefficients are substantially the same. Therefore, it ispossible to suppress an occurrence of the warpage in the substrate 8.Also, the wiring density is adjusted in the eighth layer which is thefarthest from the center line 30 (see FIG. 4), so that it is possible tocause a larger moment to suppress the warpage.

Also, the wiring density is adjusted in the eighth layer of thesubstrate 8, so that, compared to a case where the wiring density isadjusted in a layer in which a wiring layout is restricted such as asignal layer, it is possible to simplify the wiring design and reducecosts.

Also, the adhesive portion 31 is filled in the gap between adjacent NANDmemories 10, so that, by the bonding force of the adhesive portion 31,the force of drawing the NAND memories 10 occurs as shown in arrows X ofFIG. 9. This force of drawing the NAND memories 10 counteracts thewarpage force of reflexing the substrate 8 such that the first layerside has a convex shape, so that it is possible to suppress anoccurrence of warpage of the substrate 8. As long as the adhesiveportion 31 is filled, such force occurs between the controller 4 and theNAND memories 10, between the controller 4 and the DRAM 20, and betweenthe NAND memories 10 and the DRAM 20.

Further, it is provided in parts facing the gaps between the NANDmemories 10 in the wiring pattern formed over the substantially entirearea of the seventh layer of the substrate 8, so that the wiring patternbonding force in the slits 32 is weakened. Therefore, the force ofcounteracting the force (see arrows X in FIG. 9) caused by filling theadhesive portion 31 in the gap between NAND memories 10 is weakened, sothat it is possible to suppress an occurrence of warpage of thesubstrate 8 more efficiently.

Here, in the present embodiment, although a wiring layer of the eighthlayer is set to a net wiring layer to suppress the wiring density of theentire lower layer of the substrate 8, it is not limited to this, and,for example, the wiring layer may be formed on a line. Also, byadjusting the wiring densities of wiring layers in other layers than theeighth layer (i.e., fifth to seventh layers) in the lower layer, thewiring density of the entire lower layer may be adjusted. It is naturalthat the wiring density of the entire lower layer may be adjusted byadjusting the wiring densities of all layers from the fifth to eighthlayers.

Also, a layer in which the slits 32 are formed is not limited to theseventh layer. It may be formed in other layers than the seventh layerin the lower layer (i.e., fifth, sixth and eighth layers).

FIG. 11 is a view of a substrate layer configuration provided in asemiconductor memory system according to a second embodiment. In thepresent embodiment, the outermost layer is provided outside the eighthlayer of the substrate 8 as a ninth layer. Then, the entire outermostlayer is covered by copper foil to be a shield layer. Thus, by coveringthe entire outermost layer by copper foil, it is possible to preventnoise leaked from the semiconductor memory system more reliably. Also, ashield layer may be provided by covering the entire layer inside theninth layer by copper foil.

FIG. 16 is a view of a configuration example of the SATA interface 2. Inthe semiconductor memory system exemplified in the above-describedembodiment, there is a case where transmission of high-speed signals isrequired. To maintain the signal quality in the case of transmittinghigh-speed signals, it may be required to adjust the characteristicimpedance of a transmission line, optimize the cutoff frequency in thedifferential mode insertion loss characteristic and insert anappropriate choke coil in the transmission line. FIG. 14 shows anexample where choke coils 34 are inserted in the input terminals andoutput terminals of the SATA interface 2. Here, the insertion positionsof the choke coils 34 are preferably the input and output terminals ofthe SATA interface 2 but may be close to a device (such as the drivecontrol circuit 4).

FIG. 12 is an external perspective view of a holding member used in acarrying method of a semiconductor memory system according to a thirdembodiment. FIG. 13 is a cross-sectional view of a state where theholding member shown in FIG. 12 is put in a box. In the presentembodiment, the semiconductor memory system 100 is wrapped by a holdingmember 50 and then carried. The holding member 50 suppresses the warpageof the substrate 8 due to time-dependent changes.

The holding member 50 has tucking and holding portions 51 and aconnecting portion 52. Two tucking and holding portions 51 are providedfor one holding member 50. The tucking and holding portions 51 tuck andhold parts along the long-side direction of the substrate 8. To hold thesubstrate 8 on both sides, two tucking and holding portions 51 areprovided for one holding member. The tucking and holding portions 51 areformed in a cross-sectional horseshoe shape and tuck the parts along thelong-side direction of the substrate 8 therein. The tucking and holdingportions 51 suppress the warpage of the substrate 8 to counteract theforce of causing the warpage along the long-side direction of thesubstrate 8 according to time-dependent changes. Therefore, the tuckingand holding portions 51 are formed with strength to be able tocounteract the force of reflexing the substrate 8.

Also, to counteract the warpage of the substrate 8, it is preferablethat the tucking and holding portions 51 are close to the substrate 8 ina state where the substrate 8 is held. It may be configured such that,for example, the gaps formed in the tucking and holding portions 51 areformed slightly narrower than the thickness of the substrate 8 and thesubstrate 8 is inserted in the tucking and holding portions 51 withwidening the gaps. Also, it may be configured such that the widthsubstantially equal to or slightly wider than that of the substrate 8 isformed and the substrate 8 is easily inserted in the gap.

The connecting portion 52 connects the two tucking and holding portions51. By this means, it is possible to unify the holding member 50. Asshown in FIG. 13, in a case where a plurality of semiconductor memorysystems 100 are put in a box, the connecting portion 52 holds theintervals between the semiconductor memory systems 100 and alsofunctions as a shock-absorbing member for cushioning the shock given tothe semiconductor memory systems 100 at the carry time.

Here, interval holding portions 53 are formed in the tucking and holdingportions 51 individually. The interval holding portions 53 are formed soas to extend on the opposite side with respect to the tucking andholding member 51 to the side on which the connecting portions 52 areprovided. As shown in FIG. 13, in a case where a plurality ofsemiconductor memory systems 100 are put in a box, the interval holdingportions 53 hold the intervals between the semiconductor memory systems100 and also function as a shock-absorbing member for cushioning theshock given to the semiconductor memory systems 100 at the carry time.

Also, although the present embodiment has been described where thetucking and holding portions 51 tuck the substrate 8, the substrate 8 isprovided with electronic components (not shown) such as a resistance andcapacitor and the NAND memory 10. Therefore, in a case where, forexample, electronic components are provided in the surrounding part ofthe substrate 8, it is necessary to form the tucking and holdingportions 51 with widths at which it is possible to tuck the substrate 8and the electronic components collectively.

FIG. 14 is a front view of the holding member 50 according to avariation example of the third embodiment. In the present variationexample, the tucking and holding portion 51 is configured with a fixingportion 51 a and a moving portion 51 b. The fixing portion 51 a and themoving portion 51 b are connected to be rotatable in the bottom part ofthe gap formed in the tucking and holding portion 51 and be able to openand close the moving portion 51 b.

In each of the moving portions 51 b, a closure portion 55 is formed. Asshown in FIG. 14, the closure portion 55 and the moving portion 51 b arepulled toward each other when the moving portion 51 b is closed, to holda state where the moving portion 51 b is closed. Also, in the statewhere the moving portion 51 b is closed, the gap width formed in thetucking and holding portion 51 is kept constant.

FIG. 15 is a view of a state where the moving portion 51 b of theholding member 50 shown in FIG. 14 is opened. As shown in FIG. 15, byopening the moving portion 51 b, it is possible to widen the gap of thetucking and holding portion 51. In a state where the gap of the tuckingand holding portion 51 is widened, by placing the semiconductor memorysystem 100 on the fixing portion 51 a and closing the moving portion 51b, compared to a case where the semiconductor memory system 100 isinserted into the tucking and holding portion 51 with widening the gap,it is easily possible to hold the semiconductor memory system 100 in theholding member 50.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel devices described herein maybe embodied in a variety of other forms; furthermore, various omissions,substitutions and changes in the form of the devices described hereinmay be made without departing from the sprit of the inventions. Theaccompanying claims and their equivalents are intended to cover suchforms or modifications as would fall within the scope and spirit of theinventions.

What is claimed is:
 1. A semiconductor device comprising: a substrate;and a plurality of nonvolatile semiconductor memories on the substrate,wherein the substrate includes: a first metal layer having a firstpattern density, the first metal layer being electrically connected tothe nonvolatile semiconductor memories and being located on a first sideof the substrate; a second metal layer having a second pattern density,the second metal layer being located on a second side of the substrate;a plurality of third metal layers disposed between the first and secondmetal layers and having respective third pattern densities; andinsulating layers provided between the first metal layer and a first oneof the third metal layers adjacent to the first metal layer, between thesecond metal layer and a second one of the third metal layers adjacentto the second metal layer, and between adjacent ones of the third metallayers, wherein a first group of metal layers includes n metal layersincluding the first metal layer and (n−1) adjacent third metal layers, asecond group of n metal layers includes the second metal layer and (n−1)adjacent third metal layers different from the (n−1) third metal layersin the first group, a first density obtained from the first and thirdpattern densities of the first group of metal layers is equal to orgreater than a second density obtained from the second and third patterndensities of the second group of metal layers, and a difference betweenthe first density and the second density is 7.5% or less.
 2. The deviceaccording to claim 1, wherein: the first density comprises an average ofthe first pattern density and the third pattern densities of the thirdlayers in the first group of metal layers; and the second densitycomprises an average of the second pattern density and the third patterndensities of the third layers in the second group of metal layers. 3.The device according to claim 1, wherein at least one of the third metallayers comprises a shield layer.
 4. The device according to claim 1,wherein the second metal layer is a mesh layer.
 5. The device accordingto claim 4, wherein a pattern density of the mesh layer is selected toachieve said difference of 7.5% or less.
 6. The device according toclaim 1, wherein a pattern density of the second layer is selected toachieve said difference of 7.5% or less.
 7. The device according toclaim 1, wherein the first metal layer is a signal layer; a first one ofthe third metal layers in the first group of metal layers is a planelayer; third metal layers in the first group of metal layers other thanthe first one of the third metal layers are signal layers; the secondmetal layer is a mesh layer; a second one of the third metal layers inthe second group is a signal layer; and third metal layers in the secondgroup of metal layers other than the second one of the third metallayers are plane layers.
 8. The device according to claim 1, wherein atleast one of the third metal layers in the first group of metal layerscomprises a plane layer; and at least one of the third metal layers inthe second group of metal layers comprises a signal layer.
 9. The deviceaccording to claim 1, wherein the device consists of 2n layers.
 10. Thedevice according to claim 1, wherein the first density is about 60% andthe second density is about 60-67.5%.
 11. The device according to claim1, comprising slits formed in one of the layers in the second group ofmetal layers.
 12. A semiconductor device comprising: a substrate; and aplurality of nonvolatile semiconductor memories on the substrate,wherein the substrate includes: a first metal layer having a firstpattern density, the first metal layer being electrically connected tothe nonvolatile semiconductor memories and being located on a first sideof the substrate; a second metal layer having a second pattern density;a plurality of third metal layers disposed between the first and secondmetal layers and having respective third pattern densities; a shieldlayer disposed on a second side of the substrate; and insulating layersprovided between the first metal layer and a first one of the thirdmetal layers adjacent to the first metal layer, between the second metallayer and a second one of the third metal layers adjacent to the secondmetal layer, between adjacent ones of the third metal layers, andbetween the second metal layer and the shield layer, wherein a firstgroup of metal layers includes n metal layers including the first metallayer and (n−1) adjacent third metal layers, a second group of n metallayers includes the second metal layer and (n−1) adjacent third metallayers different from the (n−1) third metal layers in the first group, afirst density obtained from the first and third pattern densities of thefirst group of metal layers is equal to or greater than a second densityobtained from the second and third pattern densities of the second groupof metal layers, and a difference between the first density and thesecond density is 7.5% or less.
 13. A wiring substrate, comprising: afirst metal layer having a first pattern density, the first metal layerbeing electrically connected to a plurality of nonvolatile semiconductormemories and being located on a first side of the wiring substrate; asecond metal layer having a second pattern density, the second metallayer being located on a second side of the wiring substrate; aplurality of third metal layers disposed between the first and secondmetal layers and having respective third pattern densities; andinsulating layers provided between the first metal layer and a first oneof the third metal layers adjacent to the first metal layer, between thesecond metal layer and a second one of the third metal layers adjacentto the second metal layer, and between adjacent ones of the third metallayers, wherein a first group of metal layers includes n metal layersincluding the first metal layer and (n−1) adjacent third metal layers, asecond group of n metal layers includes the second metal layer and (n−1)adjacent third metal layers different from the (n−1) third metal layersin the first group, a first density obtained from the first and thirdpattern densities of the first group of metal layers is equal to orgreater than a second density obtained from the second and third patterndensities of the second group of metal layers, and a difference betweenthe first density and the second density is 7.5% or less.